You might call HL customer service with your trailer ID number along with your make and model. They should have the answer. I would also think if you went to the Winegard web sight you might look for a installation instructions.
I have used Simple Green in the past with no problems. If you use it be sure to rinse the sides of the trailer also as you rinse the roof. Simple green can remove the wax if you leave it too long on the sides.
It sounds like the previous owner who lived in it might have left the valves open all the time. I am presuming they were stationary all the time. This can cause all kinds of problems. You might try back flushing the tanks so see if that helps free up some crap in the tanks.
On a Hitchiker I owned, the factory did not apply the glue correctly around the front a/c which caused water to leak in slowly. Eventually the glue loosened up the front roof material causing a lot of damage. What was nice is that they admitted it was their fault and paid the $9200 to the...